The present invention relates to certain aqueous polymer dispersions containing a polyurethane polymer and a vinyl polymer.
Aqueous polyurethane dispersions which are used as or provide the basis of aqueous-based coating compositions are well known for the production of coatings on various substrates. They may be used for the provision of e.g. protective or decorative coatings since polyurethanes, depending on their particular composition, can possess many desirable properties such as good chemical resistance, water-resistance, solvent resistance, toughness, abrasion-resistance, substrate adhesion and durability. Dispersion of the polyurethane in the aqueous system has been achieved either by the use of external surfactants or, more usefully, by including appropriate chain-pendant ionic and/or nonionic groups in the structure of the polyurethane polymer. In the latter respect, such aqueous polyurethane dispersions are particularly advantageously prepared (as is by now well-known in the art) by dispersing an isocyanate-terminated polyurethane prepolymer bearing ionic and/or nonionic dispersing groups into an aqueous medium and reacting the prepolymer with an active hydrogen-containing chain extender during and/or after dispersion in the aqueous medium. In an alternative well known process, known as the xe2x80x9cacetonexe2x80x9d process, the prepolymer is chain-extended while dissolved in an organic solvent (usually acetone) followed by the addition of water until water becomes the continuous phase and the subsequent removal of the solvent.
It is also now well known to modify the properties of polyurethane coatings derived from aqueous dispersions thereof by incorporating vinyl polymers, and in particular acrylic polymers, into the dispersions. For example, the use of acrylic polymers may allow upgrading of the coatings by increasing their hardness. Such dispersions may include the polyurethane and vinyl polymers as a simple blend of the preformed polymer dispersions. However, several patents, for example U.S. Pat. Nos. 3,705,164, 4,198,330 and 4,318,833, EP 0189945 and EP 0308115, describe processes where the vinyl polymer is formed in situ by polymerising one or more vinyl monomers in the presence of an aqueous dispersion of a polyurethane containing anionic salt groups. Such in-situ formation of the vinyl polymer can be advantageous in that it may result in greater dispersion stability and may further improve the performance of the resulting coating in comparison to simple blending of the polyurethane and vinyl polymers.
We have now discovered certain of these aqueous polyurethane/vinyl polymer dispersions (the vinyl polymer being incorporated as a simple blend or being present as a preformed polymer when the polyurethane prepolymer is chain extended, or more preferably being formed in-situ) which provide coatings of exceptionally good block resistance and exceptionally good imprint resistance. This is especially advantageous in many wood and board (i.e. compressed and glued wood particles or fibres) coating applications, for which the invention aqueous polymer dispersions are particularly useful, where the avoidance of coated substrates tending to stick together (i.e. block), as e.g. in coated window and door frames or in stacked coated boards (during storage), is highly desirable. In addition, the invention polymer dispersions provide coatings having good resistance to many chemicals and solvents including e.g. household chemicals. Still further, the urethane component of the dispersions, which are of relatively high acid value (see following), may significantly depress the minimum film forming temperatures (MFT) of high Tg vinyl polymers which may be used in the dispersion, without reducing the hardness. Also, the in situ prepared urethane/vinyl polymer dispersions show a favourable hardness/MFT balance.
According to the present invention there is provided an aqueous polymer dispersion containing a water-dispersed polyurethane polymer and a vinyl polymer in a weight ratio of from 30/70 to 5/95 wherein said polyurethane polymer is the reaction product of:
A) an isocyanate-terminated polyurethane prepolymer having an acid value of xe2x89xa744 mg KOH/g of prepolymer (solids) and formed from reactants comprising an organic polyisocyanate component and an organic poly(isocyanate-reactive group) component in which the isocyanate-reactive groups are selected from xe2x80x94OH and optionally one or more of xe2x80x94NH2, xe2x80x94NHxe2x80x94, and xe2x80x94SH, wherein
i) at least 70 weight % of said polyisocyanate component is constituted by at least one aliphatic polyisocyanate,
ii) said poly(isocyanate-reactive group) component includes at least one acid-bearing poly(isocyanate-reactive group) compound for providing anionic groups which provide or contribute to water-dispersibility,
iii) the reactants are used in amounts corresponding to a ratio of isocyanate groups to isocyanate-reactive groups within the range of 1.4/1 to 2.9/1; and
B) an active hydrogen chain-extending compound(s).
There is further provided according to the invention a process for the production of an aqueous polymer dispersion containing a water-dispersed polyurethane polymer and a vinyl polymer, wherein said process comprises:
I synthesising an isocyanate-terminated polyurethane prepolymer having an acid value of xe2x89xa744 mg KOH/g of prepolymer (solids) from reactants comprising an organic polyisocyanate component and an organic poly(isocyante-reactive group) component in which the isocyanate-reactive groups are selected from xe2x80x94OH and optionally one or more of xe2x80x94NH2, xe2x80x94NHxe2x80x94, and xe2x80x94SH; wherein
(i) at least 70 weight % of said polyisocyanate component is constituted by at least one aliphatic polyisocyanate,
(ii) said poly(isocyanate-reactive group) component includes at least one acid-bearing poly(isocyanate-reactive group) compound for providing anionic groups which provide or contribute to water-dispersibility,
(iii) the reactants are used in amounts corresponding to a ratio of isocyanate groups to isocyanate-reactive groups within the range of 1.4/1 to 2.9/1;
II chain extending the polyurethane prepolymer using an active hydrogen chain extending compound(s) to form a polyurethane polymer;
III forming an aqueous dispersion of said polyurethane polymer;
IV incorporating a vinyl polymer into said polyurethane polymer dispersion, whereby the weight ratio of the polyurethane polymer to the vinyl polymer is within the range of from 30/70 to 5/95.
The stages II, III and IV of the process are not necessarily carried out sequentially in the order shown or as individual steps (i.e. any desired order or regime can be used), as will become apparent later. (For example, the chain extension step may be carried out simultaneously with the formation of the aqueous polyurethane polymer dispersion by dispersion of the polyurethane prepolymer into an aqueous medium containing the chain-extender, or into an aqueous medium into which the chain-extender is subsequently incorporated; also, in such embodiments, rather than, as is usual, effecting vinyl polymerisation subsequent to chain-extension it would in principle be be possible to disperse the prepolymer into an aqueous dispersion of a preformed vinyl polymer with simultaneous or subsequent chain extension of the prepolymer. Use of the xe2x80x9cacetonexe2x80x9d process, however, will normally entail stages II and III being separate steps and performed sequentially. It might also be possible, for example, to carry out stages II, III and IV simultaneously).
There is further provided according to the invention a coating obtainable or derived from an aqueous composition comprising an aqueous polymer dispersion as defined above.
There is further provided according to the invention a method of coating a substrate using an aqueous composition comprising an aqueous polymer dispersion as defined above.
There is further provided according to the invention a substrate having a coating obtainable or derived from an aqueous composition comprising an aqueous polymer dispersion as defined above.
(It is to be understood that the vinyl polymer component of the invention dispersion could in fact be more than one vinyl polymer, e.g. 2 or more vinyl polymers, such as might be formed by a multi-stage vinyl polymerisation processxe2x80x94see later).
For the purposes of this invention an xe2x80x9caqueous dispersionxe2x80x9d means a dispersion of the polymers in a liquid carrier medium of which water is the principle component (at least 50 weight %, more usually at least 80 weight %, of the carrier medium). Minor amounts of organic liquids may optionally be present. The dispersion will typically comprise colloidally dispersed particles of the polyurethane and vinyl polymers, i.e. will typically be in the form of an aqueous latex.
It has thus been discovered that aqueous polymer dispersions according to the invention provide coatings of exceptionally good block resistance and exceptionally good imprint resistance, the high block resistance and high imprint resistance surprisingly being found not to be a mere consequence of the hardness of the coating or of the glass transition temperature of the vinyl polymer component (as might have been expected). Further, as mentioned above, the invention dispersions provide coatings of good resistance to many chemicals and solvents including, e.g. household chemicals.
It is essential that at least 70 weight % of the polyisocyanate component used in making the polyurethane prepolymer is an aliphatic polyisocyanate(s), this term (for the sake of clarity) being intended to mean compounds in which all the isocyanate groups are directly bonded to aliphatic or cycloaliphatic groups, irrespective of whether aromatic groups are also present. More preferably the amount of aliphatic polyisocyanate is at least 85 weight %, and most preferably 100 wt %, of the polyisocyanate component.
Examples of suitable aliphatic polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane-1,4-diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate, cyclopentylene diisocyanate, p-tetra-methylxylene diisocyanate (p-TMXDI) and its meta isomer (m-TMXDI), hydrogenated 2,4-toluene diisocyanate, hydrogenated 2,6-toluene diisocyanate, and 1-isocyanato-1-methyl-3(4)-isocyanatomethyl-cyclohexane (IMCI).
Suitable non-aliphatic polyisocyanates (if used) include p-xylylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4xe2x80x2-diphenylmethane diisocyanate, and 1,5-naphthylene diisocyanate.
Mixtures of polyisocyanates can be used and also polyisocyanates which have been modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues.
Preferred aliphatic polyisocyanates are cycloaliphatic polyisocyanates such as 4,4xe2x80x2-dicyclohexylmethane diisocyanate.
The organic poly(isocyanate-reactive group) component used in the synthesis of the polyurethane prepolymer will normally consist essentially of an organic polyol component, i.e. a component consisting essentially of at least one organic polyol. Nevertheless it is within the scope of the invention for the organo poly(isocyanate-reactive group) component to comprise in addition to, or as an alternative to, an organic polyol(s), a poly(isocyanate-reactive group) compound(s) which is other than a polyol(s)xe2x80x94provided of course that at least some of the isocyanate-reactive groups of the poly(isocyanate-reactive groups) component are hydroxyl groups. Examples of such other compounds include compounds in which one or more of the isocyanate-reactive groups are primary or secondary amino groups, such as ethanolamine. Other examples include 3-amino-2,2-dimethylpropane-1-ol (neopentanolamine), 2-amino-1-butanol, monoisopropylamine, N-methylethanolamine, and in general Michael adducts from 1 mole of a primary amine with 2-hydroxyethyl acrylate or Michael adducts from ethanolamine and acrylic monomers.
The acid-bearing poly(isocyanate-reactive group) compound(s) used in the production of the polyurethane prepolymer is normally an acid bearing polyol(s), and is preferably a low molecular weight ( less than 500) carboxylic acidxe2x80x94bearing polyol, in particular a diol or triol, whereby carboxylate anion groups are provided by the carboxylic acid groups. (While normally being used directly in the prepolymer synthesis, i.e. as a separate component, it is within the scope of the invention, although much less preferred, for it to be first incorporated into a higher molecular weight polyol, e.g. a polyester polyol, e.g. by reaction with a dicarboxylic acid component used in the higher molecular weight polyol preparation). Particularly preferred are dihydroxyalkanoic acids of formula 
where R1 is hydrogen or alkyl (usually 1-5 C). By far the most preferred carboxyl-bearing polyol is 2,2-dimethylol propionic acid (DMPA).
The conversion of the acid groups present in the prepolymer to anionic salt groups may be effected (where necessary) by neutralising the acid groups with a suitable base such as ammonia or triethylamine.
A critical feature of the invention is that the acid value of the prepolymer must be xe2x89xa744 mg KOH/g prepolymer. Below this level, poor block and imprint resistance tends to result. A preferred upper level for the acid value of the prepolymer is 100 mg KOH/g prepolymer. A more preferred range for prepolymer acid value is from 48 to 80 mg KOH/g, most preferably 56-70 mg KOH/g.
Where DMPA is employed as the acid-bearing polyol(s), as is most preferred, a prepolymer acid value of 44 mg KOH/g corresponds to an amount of ca 10.6% by weight of DMPA in the prepolymer reactants.
Another critical feature of the invention is that the prepolymer reactants are used in amounts corresponding to a ratio of isocyanate groups to isocyanate-reactive groups within the range of 1.4/1 to 2.9/1. In the case when the poly(isocyanate-reactive group) component consists essentially of an organic polyol component, i.e. is either entirely an organic polyol(s) or if not entirely are organic polyol(s) then any organic poly(isocyanate-reactive group) compound(s) other than a polyol(s) used in the synthesis having no material effect on the properties of the resulting product, then the above-mentioned critical feature of the invention corresponds to a ratio of isocyanate groups to hydroxyl groups within the range of 1.4/1 to 2.9/1.
Below a ratio of 1.4/1 poor block resistance tends to result, even if the acid value is xe2x89xa744. Above a ratio of 2.9/1, an excessively hard blocky material is obtained, which is in any case is very expensive to produce (because of the high isocyanate content) and will contain much unreacted isocyanate, making the prepolymer difficult to disperse into water. More preferably the isocyanate/isocyanate-reactive group (normally hydroxyl) ratio is from 1.6/1 to 2.5/1.
The poly(isocyanate-reactive group) component (normally a polyol component) for making the prepolymer will almost always include one or more poly(isocyanate-reactive group) compounds (normally a polyol(s)) other than an acid-bearing poly(isocyanate-reactive group) compound(s).
In particular a polymeric polyol(s) of molecular weight from 500 to 6000 is normally present, and optionally a low molecular weight polyol(s) of molecular weight below 500 (i.e. in addition to acid-bearing compounds of the type discussed above).
Polymeric polyols having molecular weights in the range of 500-6000 which may be used in the preparation of the prepolymer particularly include diols and triols and mixtures thereof but higher functionality polyols may be used for example as minor components in admixture with diols. The polyols may be members of any of the chemical classes of polymeric polyols used or proposed to be used in polyurethane formulations. In particular, the polyols may be polyesters, polyesteramides, polyethers, polythioethers, polycarbonates, polyacetals, polyolefins or polysiloxanes. Preferred polyol molecular weights are from 700 to 3000.
Polyester polyols which may be used include hydroxyl-terminated reaction products of polyhydric alcohols as ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, 1,6-hexanediol, furan dimethanol, cyclohexane dimethanol, glycerol, trimethylolpropane or pentaerythritol, or mixtures thereof, with polycarboxylic acids, especially dicarboxylic acids or their ester-forming derivatives, for example succinic, glutaric and adipic acids or their methyl esters, phthalic anhydride or dimethyl terephthalate. Polyesters obtained by the polymerisation of lactones, for example caprolactone, in conjunction with a polyol may also be used. Polyesteramides may be obtained by the inclusion of amino-alcohols such as ethanolamine in polyesterification mixtures.
Polyether polyols which may be used include products obtained by the polymerisation of a cyclic oxide, for example ethylene oxide, propylene oxide or tetrahydrofuran or by the addition of one or more such oxides to polyfunctional initiators, for example water, ethylene glycol, propylene glycol, diethylene glycol, cyclohexane dimethanol, glycerol, trimethylopropane, pentaerythritol or Bisphenol A. Especially useful polyesters include polyoxypropylene diols and triols, poly (oxyethylene-oxypropylene) diols and triols obtained by the simultaneous or sequential addition of ethylene and propylene oxides to appropriate initiators and polytetramethylene ether glycols obtained by the polymerisation of tetrahydrofuran.
Polythioether polyols which may be used include products obtained by condensing thiodiglycol either alone or with other glycols, dicarboxylic acids, formaldehyde, aninoalcohols or aminocarboxylic acids.
Polycarbonate polyols which may be used include products obtained by reacting diols such as 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol or tetraethylene glycol with diaryl carbonates, for example diphenyl carbonate, or with phosgene, or cycloaliphatic carbonates such as ethylene carbonate or propylene carbonate; dialkyl carbonate such as diethyl carbonate, can also be used.
Polyacetal polyols which may be used include those prepared by reacting glycols such as diethylene glycol, triethylene glycol or hexanediol with formaldehyde. Suitable polyacetals may also be prepared by polymerising cyclic acetals.
Suitable polyolefin polyols include hydroxy-terminated butadiene homo and copolymers.
Polyols having molecular weights below 500 which may optionally be used in the preparation of the prepolymer particularly include diols and triols and mixtures thereof but higher functionality polyols may be used. Examples of such lower molecular weight polyols include ethylene glycol, diethylene glycol, tetraethylene glycol, bis (hydroxyethyl) terephthalate, 1,4-cyclohexane dimethanol, furan dimethanol, glycerol and the reaction products, up to molecular weight 499, of such polyols with propylene and/or ethylene oxide.
The reactants to form the polyurethane prepolymer may optionally also include isocyanate-reactive materials bearing nonionic groups for introducing nonionic dispersing groups into the prepolymer structure. Such groups may e.g. be provided by employing as a reactant in the prepolymer formation a polyol (preferably a diol) having pendant polyoxyethylene chains such as those described in the prior art, e.g. U.S. Pat. No. 3,905,925. If present, the nonionic dispersing group content will usually be present in the range 0.5 to 25 g of nonionic dispersing groups (particularly polyethylene oxide groups) per 100 g polyurethane prepolymer.
The isocyanate-terminated polyurethane prepolymer may be prepared in conventional manner by reacting the organic polyisocyanate and poly (isocyanate-reactive group) components and any other reactants under substantially anhydrous conditions at a temperature between about 30xc2x0 C. and about 130xc2x0 C. until reaction between the isocyanate groups and the isocyanate-reactive (usually hydroxyl) groups is substantially complete.
If desired, catalysts such as dibutylin dilaurate or stannous octoate may be used to assist prepolymer formation. An organic solvent may optionally be added before, during or after prepolymer formation to control the viscosity. Suitable solvents which may be used include acetone, methylethylketone, dimethylformamide, diglyme, N-methylpyrrolidone, ethyl acetate, ethylene and propylene glycol diacetates, alkyl ethers of ethylene and propylene glycol diacetates, alkyl ethers of ethylene and propylene glycol monoacetates, toluene, xylene and sterically hindered alcohols such as t-butanol and diacetone alcohol. The preferred solvents are water-miscible solvents such as N-methylpyrrolidone, acetone and dialkyl ethers of glycol acetates or mixtures of N-methylpyrrolidone and methyl ethyl ketone.
The aqueous polyurethane dispersion is preferably prepared by forming an aqueous dispersion of the isocyanate-terminated polyurethane prepolymer by dispersing it (optionally carried in an organic solvent medium) in an aqueous medium, preferably utilising the self-dispersibility properties of the prepolymer arising from the carboxylate anion groups (and nonionic groups if present) although free surfactant(s) may additionally be employed if desired, and chain extending the prepolymer with an active hydrogen-containing chain extender in the aqueous phase, the chain extender being present in the aqueous phase during dispersion or added subsequently (i.e. chain-extension can take place during and/or after the dispersion into water in this embodiment).
The prepolymer may be dispersed in water using techniques well known in the art. Preferably, the prepolymer is added to the water with agitation or, alternatively, water may be stirred into the prepolymer.
Alternatively, although less preferably, the prepolymer may be chain extended to form the polyurethane polymer while dissolved in an organic solvent (usually acetone) followed by the addition of water to the polymer solution until water becomes the continuous phase and the subsequent removal of solvent (e.g. by distillation) to form an aqueous dispersion (the well-known xe2x80x9cacetone processxe2x80x9d).
The active hydrogen-containing chain extender compound(s) which may be reacted with the prepolymer is suitably a polyol, an amino-alcohol, a primary or secondary aliphatic, alicyclic, aromatic, araliphatic or heterocyclic diamine or polyamine, or hydrazine or a substituted hydrazine. Water-soluble chain extenders are preferred.
Water itself may be used as an indirect chain-extender (although not when using the xe2x80x9cacetone processxe2x80x9d) because it will slowly convert some of the terminal isocyanate groups of the prepolymer to amino groups and the modified prepolymer molecules will than act as a chain-extender for the unreacted isocyanate-terminated prepolymer molecules. However this is very slow compared to chain-extension using the above mentioned active hydrogen chain extenders which will provide the predominant chain-extension reaction if used.
Examples of such chain extenders useful herein include ethylene diamine, diethylene triamine, triethylene tetramine, propylene diamine, butylene diamine, hexamethylene diamine, cyclohexylene diamine, piperazine, 2-methyl piperazine, phenylene diamine, tolylene diamine, xylylene diamine, tris (2-aminoethyl) amine, 3,3-dinitrobenzidine, 4,4xe2x80x2-diaminodiphenylmethane, methane diamine, m-xylene diamine, isophorone diamine, and adducts of diethylene triamine with acrylate or its hydrolysed products. Also materials such as hydrazine (e.g. in the form of its mono hydrate), azines such as acetone azine, substituted hydrazines such as, for example, dimethyl hydrazine, 1,6-hexamethylene-bis-hydrazine, carbodihydrazine, hydrazides of dicarboxylic acids and sulphonic acids such as adipic acid dihydrazide, oxalic acid dihydrazide, isophthalic acid dihydrazide, hydrazides made by reacting lactones with hydrazine such as gamma-hydroxylbutyric hydrazide, bis-semi-carbazide, and bis-hydrazide carbonic esters of glycols.
Where the chain extender is other than the modified prepolymer molecules formed by reaction with water (assuming the prepolymer dispersion into water embodiment is being used), for example a polyamine or hydrazine, it may be added to the aqueous dispersion of prepolymer or, alternatively, it may already be present in the aqueous medium when the prepolymer is dispersed therein.
The chain extension can be conducted at elevated, reduced or ambient temperatures. Convenient temperatures are from about 5xc2x0 C. to 90xc2x0 C., more preferably 10 to 60xc2x0 C.
The total amount of chain extender material(s) employed (other than water) should be approximately equivalent to the free-NCO groups in the prepolymer, the ratio of active hydrogens in the chain extender(s) to NCO groups in the prepolymer preferably being in the range from 0.7/1 to 2.0/1 more preferably 0.85/1 to 1.2/1. Of course, when water is employed as an indirect chain extender, these ratios will not be applicable since the water, functioning both as an indirect chain extender and a dispersing medium, will be present in a gross excess relative to the free-NCO groups.
The resulting polyurethane polymer (after chain extension) will often have a number average weight within the range of 10,000 to 300,000 daltons.
[It is evident from the foregoing that the term xe2x80x9cpolyurethanexe2x80x9d as used in this specification is intended to apply not only to polymers (or prepolymers) having only urethane linkages formed from isocyanate and hydroxyl groups, but optionally also to polymers (or prepolymers) having, in addition to urethane linkages, linkages formed from isocyanate and xe2x80x94NH2, xe2x80x94NHxe2x80x94, or xe2x80x94SH groups].
The polyurethane and vinyl polymers may be brought together by any suitable technique.
For example, an aqueous dispersion of the polyurethane and an aqueous dispersion of the vinyl polymer, separately prepared, may merely be blended together (with agitation if necessary). In another method, the polyurethane prepolymer could in principle be dispersed into an aqueous dispersion of a preformed vinyl polymer with simultaneous or subsequent chain extension of the prepolymer.
More preferred, however, is to perform the preparation of the vinyl polymer in-situ in the presence of the polyurethane polymer during and/or after its formation. In such an embodiment the vinyl monomer(s) for making the vinyl polymer may be introduced in the process at any suitable stage. For example, where an aqueous dispersion of the prepolymer is formed in the process to make the polyurethane polymer (as is preferred) all of the vinyl monomer(s) may be added to the prepolymer prior to its dispersion into water, or all of the vinyl monomer(s) may be added subsequent to dispersion (or may have already been added to the water prior to the dispersion of the prepolymer therein), or part of the monomer(s) may be added to the prepolymer prior to dispersion and the remainder added subsequent to dispersion (or the remainder may have already been added to the water prior to the dispersion of the prepolymer therein). In the case where all or part of the monomer(s) is added to the prepolymer prior to dispersion into water, such monomer(s) could be added to the prepolymer subsequent to its formation or prior to its formation, or some could be added subsequent to its formation and some added prior to its formation. In the case where any vinyl monomer(s) is added prior to the prepolymer formation it may provide at least part of a solvent system for the reaction to form the prepolymer (if it possesses suitable solvent characteristics).
The vinyl polymer of the invention composition is normally made by an aqueous free-radical polymerisation process. When made in-situ, and where an aqueous dispersion of the polyurethane prepolymer is formed in the process to make the polyurethane polymer, with chain extension being carried out in the aqueous phase (as is preferred), the vinyl polymerisation may be performed simultaneously with the chain extension step, or performed subsequent to the chain extension step, or performed partly simultaneously with the chain-extension step and partly subsequent to the to the chain extension step. If the xe2x80x9cacetone processxe2x80x9d is adopted for making the polyurethane polymer (see above for a discussion of this technique) wherein chain extension occurs in an organic solvent phase (usually acetone), the vinyl polymerisation is normally performed subsequent to the chain extension step after removal of solvent to form an aqueous dispersion of the polyurethane polymer (although it would, in principle, be possible to perform the vinyl polymerisation in the solvent, disperse the urethane/vinyl polymer into water, and then remove the solvent).
All of the vinyl monomer(s) to be polymerised may be present at the commencement of the vinyl polymerisation, or alternatively in cases where all or part of the monomer(s) to be polymerised has been introduced subsequent to the formation of an aqueous prepolymer dispersion, or, in the case of using the xe2x80x9cacetone processxe2x80x9d, normally subsequent to the formation of the chain extended polyurethane aqueous dispersion (although it would, in principle, be possible to add the vinyl monomer(s) at the prepolymer formation stage), some or all of that monomer(s) may be added to the reaction medium during the course of the polymerisation (in one or more stages or continuously).
By a vinyl polymer herein is meant a homo or copolymer derived from the addition polymerisation (using a free radical initiated process and usually in an aqueous medium) of one or more olefinically unsaturated monomers. If the vinyl polymer has been made using an in-situ technique (see above) it preferably has an acid value of xe2x89xa615 mg KOH/g. A high acid content in the in-situ-formed vinyl polymer would tend to destabilise the resulting dispersion. Particularly preferred vinyl polymers are acrylic polymers (i.e. based predominantly on at least one ester of acrylic or methacrylic acid).
It is to be understood that in some embodiments it is possible for two or more vinyl polymers (usually two, as formed e.g. in a sequential or multistage vinyl polymerisation process), instead of just one vinyl polymer, to be present in the dispersion (this being a commonly understood equivalence) so that the term xe2x80x9cvinyl polymerxe2x80x9d extends to 1 or more vinyl polymers.
The polymerisation of the at least one vinyl monomer to form the vinyl polymer will require the use of a free-radical-yielding initiator(s) to initiate the vinyl polymerisation. Suitable free-radical-yielding initiators include inorganic peroxides such as K, Na or ammonium persulphate, hydrogen peroxide, or percarbonates; organic peroxides, such as acyl peroxides including e.g. benzoyl peroxide, alkyl hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide; dialkyl peroxides such as di-t-butyl peroxide; peroxy esters such as t-butyl perbenzoate and the like; mixtures may also be used. The peroxy compounds are in some cases advantageously used in combination with suitable reducing agents (redox systems) such as Na or K pyrosulphite or bisulphite, and i-ascorbic acid. Azo compounds such as azoisobutyronitrile may also be used. Metal compounds such Fe. EDTA (EDTA is ethylene diamine tetracetic acid) may also be usefully employed as part of the redox initiator system. We particularly prefer to use an initiator system partitioning between the aqueous and organic phases, e.g. a combination of t-butyl hydroperoxide, iso-ascorbic acid and Fe.EDTA. The amount of initiator or initiator system to use is conventional, e.g. within the range 0.05 to 6 wt % based on the total vinyl monomer(s) used.
An aqueous vinyl polymerisation carried out in the absence of the polyurethane normally needs to be performed in the presence of a stabilising and/or dispersing material, and when making an aqueous latex of a vinyl polymer, a conventional emulsifying agent would need to be employed (e.g. anionic and/or non-ionic emulsifiers such as Na salts of dialkylsulphosuccinates, Na salts of sulphated oils, Na salts of alkyl sulphonic acids, Na, K and ammonium alkyl sulphates, C22-24 fatty alcohols, ethyoxylated fatty acids and/or fatty amides, and Na salts of fatty acids such as Na stearate and Na oleate; the amount used is usually 0.1 to 5% by weight on the weight based on the total olefinically unsaturated monomer(s) used). When incorporated using an in-situ process, however, the polyurethane polymer containing anionic (and optionally nonionic) dispersing groups usually removes the requirement for the use of a separately added conventional emulsifying agent since the polyurethane itself acts as an effective dispersant for the vinyl polymerisation, although a conventional emulsifier can be still employed if desired.
Examples of vinyl monomers which may be used to form the vinyl polymer include 1,3-butadiene, isoprene, styrene, xcex1-methyl styrene, divinyl benzene, acrylonitronitrile, methacrylonitrile, vinyl halides such as vinyl chloride, vinyl esters such as vinyl acetate, vinyl propionate, vinyl laurate, and vinyl esters of versatic acid such as VeoVa 9 and VeoVa 10 (VeoVa is a trademark of Shell), heterocyclic vinyl compounds, alkyl esters of mono-olefinically unsaturated dicarboxylic acids (such as di-n-butyl maleate and di-n-butyl fumarate) and, in particular, esters of acrylic acid and methacrylic acid of formula
CH2xe2x95x90CR2COOR3
wherein R2 is H or methyl and R3 is optionally substituted alkyl or cycloalkyl of 1 to 20 carbon atoms (more preferably 1 to 8 carbon atoms) examples of which are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-propyl acrylate, n-propyl methacrylate, and hydroxyalkyl (meth)acrylates such as hydroxyethyl acrylate, hydroxyethylmethacrylate 2-hydroxypropyl methacrylate, 2-hydroxypropyl acrylate, and their modified analogues like Tone M-100 (Tone is a trademark of Union Carbide Corporation). Olefinically unsaturated monocarboxylic and/or dicarboxylic acids, such as acrylic acid, methacrylic acid, fumaric acid, and itaconic acid, are other examples which can be used, but in the case of making the vinyl polymer using an in-situ technique should preferably only be employed to provide a small level of copolymerised units in the polymer such that the acid value of the resulting vinyl polymer is xe2x89xa615 mg KOH/g. More preferably acid-bearing vinyl monomer(s) is not employed at all for in-situ formed vinyl polymers. When the vinyl polymer is preformed (and e.g. incorporated by simple blending), the acid level need not be preferably limited in this way.
Particularly preferred are vinyl polymers made from a monomer system comprising at least 40 weight % of one or more monomers of the formula CH2xe2x95x90CR2COOR3 defined above. Such preferred polymers are defined herein as acrylic polymers. More preferably, the monomer system contains at least 50 weight % of such monomers, and particularly at least 60 weight %. The other monomers in such acrylic polymers (if used) may include one or more of the other vinyl monomers mentioned above, and/or may include ones different to such other monomers. Styrene is a useful other monomer. Preferred (meth)acrylic ester monomers include methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate.
The glass transition temperature (Tg) of the vinyl polymer may vary within a wide range, a possible range being from xe2x88x9250 to 120xc2x0 C. In a preferred embodiment, a multistage process may be formed to form the vinyl polymer (as mentioned above), so that there will be, in effect, two (or more) vinyl polymers formed having two or more Tg""s of which, more preferably, at least two Tg""s differ by at least 30xc2x0 C., more preferably at least 60xc2x0 C. The number average molecular weight of the or each vinyl polymer will often be in the range of from 10,000 to 300,000 daltons.
The weight ratio of the polyurethane polymer component to the vinyl polymer component should be within the range of from 30/70 to 5/95, more preferably from 25/75 to 10/90.
The aqueous dispersions of the invention typically have a solids content of from about 20 to 60% by weight, more usually from 25 to 50% by weight.
The aqueous dispersions of the invention are particularly useful as or for providing the principle component of coating compositions (e.g. protective, decorative or adhesive coating compositions) for which purpose they may be further diluted with water and/or organic solvents, or they may be supplied in more concentrated form by evaporation of water and/or organic components of the liquid medium. As coating compositions, they may be applied to a variety of substrates including wood, board, metals, stone, concrete, glass, cloth, leather, paper plastics, foam and the like, by any conventional method including brushing, dipping, flow coating, spraying, and the like. They are, however, particularly useful for providing coatings on wood and board substrates. The aqueous carrier medium is removed by natural or accelerated (by heat) drying to form a coating. The compositions may contain other conventional ingredients including coalescing organic solvents, pigments, dyes, emulsifiers, surfactants, thickeners, heat stabilisers, levelling agents, anti-cratering agents, fillers, sedimentation inhibitors, UV absorbers, antioxidants and the like introduced at any stage of the production process or subsequently. It is possible to include an amount of antimony oxide in the dispersions to enhance the fire retardant properties.
If desired the aqueous dispersion of the invention can be used in combination with other polymer dispersions or solutions which are not according to the invention.